Mouse microphthalmia transcription factor (Mitf) mutations affect the development of four cell types: melanocytes, mast cells, osteoclasts, and pigmented epithelial cells of the eye. The mutations are phenotypically diverse and can be arranged in an allelic series. In humans, MITF mutations cause Waardenburg syndrome type 2A (WS2A) and Tietz syndrome, autosomal dominant disorders resulting in deafness and hypopigmentation. Mitf mice thus represent an important model system for the study of human disease. During the past year, we described the complete exon/intron structure of the mouse Mitf gene and showed it to be similar to the human gene. We also showed that the mouse gene is more transcriptionally complex than previously thought and is capable of generating at least 13 different isoforms. Some of these isoforms are missing important functional domains of the protein, suggesting that they might play an inhibitory role in Mitf function and signal transduction. In addition, we determined the molecular basis for six microphthalmia alleles. Two of the mutations were described for the first time (Mitf(mi-enu198) and Mitf(mi-x39)), while the others (Mitf(mi-ws), Mitf(mi-bws), Mitf(mi-ew), and Mitf(mi-di)) have been described but the molecular basis for the mutation not determined. When analyzed in terms of the genomic and transcriptional data we obtained, it is apparent that these mutations result from RNA processing or transcriptional defects. Interestingly, three of the mutations (Mitf(mi-x39), Mitf(mi-bws), and Mitf(mi-ws)) produce proteins that are missing important functional domains of the protein identified in in vitro studies, further confirming a biological role for these domains in the whole animal. The osteoclast defects observed in Mitf mice are only seen in homozygous animals carrying strong dominant Mitf alleles. The MITF protein, a basic-helix-loop-helix-leucine zipper (bHLHZip) transcription factor, forms heterodimers with the related TFE3, TFEB, and TFEC proteins. Studies of the MYC/MAX/MAD bHLHZip proteins have suggested that the functional effects of the different family members are chiefly mediated by heterodimers. Very recently, we have shown that the importance of heterodimeric interactions varies dramatically between different bHLHZip subfamilies in vivo. While loss of either the Mitf or Tfe3 genes alone does not lead to osteopetrosis, their combined loss results in severe osteopetrosis. No additional effects on other cell types can be detected and neither Tfeb nor Tfec contribute to the phenotype, suggesting that this partnership is specific to Mitf and Tfe3 in osteoclasts. Furthermore, our results suggest that some forms of human osteopetrosis may be due to mutations at multiple loci where the resulting phenotype is both dose and allele dependent.